The present invention relates to an optical logic element for controlling an optical signal by light, which is indispensable to high-speed optical signal processings in the fields of optical switching and optical information processings.
It is now being expected that wide-band, new services, such as moving picture communication and image distribution, are developed extensively through utilization of the optical fiber communication technology which permits ultrawide band, ultrahigh-speed transmission. In this instance, ultrahigh-speed signal processing is indispensable at a node at which wide-band signals center. To meet with this requirement, particular attention is being directed to a so-called optical switching, optical signal processing system which processes optical signals intact or processes them making the most use of properties of light and which can therefore be expected to allow a high-speed switching operation and afford further reduction of the processing time through parallel processing, in comparison with a system which once converts optical signals to electrical signals and processes them by the use of an LSI or similar electronic circuit.
One of important functions in optical signal processing is a signal identifying function for identifying an input optical signal to provide it to a desired path. Usually, a signal composed of a digital sequence uses its bit pattern as an indication for recognition, and it can be said that the signal identifying function is to perform a bit pattern matching operation. The bit pattern matching means a function which determines whether digital signal bits of a plurality of input signal cells, allotted to each time slot, match or not, thereby ultimately determining whether the cells match or not. To inplement such a function, an optical logic element is needed which performs an exclusive-OR (XOR) or exclusive-NOR (XOR) operation as a logical operation which provides a different output signal, depending on whether all the above-mentioned plurality of bits assume the same states "1" (or "0") or not.
FIG. 9 shows a conventional XOR optical logical element. Reference numerals 100 and 100' denote photo-transistors (HPT's) each composed of n-InP, p-InGaAsP and n-Inp layers and 101 and 101' LED's each composed of n-Inp, InGaAsP and p-InP layers. The LED's are each connected in series to one of the two HPT's which are simultaneously irradiated with input light beams A and B, and two such unit cells are connected in parallel to the power supply. FIG. 10 shows a cross-sectional structure of the unit cell surrounded by a one-dot chain line. Reference numeral 102 indicates a semi-insulating InP layer, 103 an n-InP layer, 104 a p-InGaAsP layer, 105 an n-InP layer, 106 an InGaAsP layer, 107 a p-InP layer, 108 a p-InGaAsP layer, 109 an Au-Zn layer, 110 an Au-Sn layer, 111 a polyimide layer, and 112 a Ti/Au layer. The InGaAsP layers 104 and 106 correspond to a base layer of each HPT and a light emitting layer of each LED, respectively. Assuming that only the input light A (or B) is applied as shown in FIG. 9, the HPT 100 (or 100') are turned ON and the HPT 100' (or 100) connected in parallel thereto are turned OFF, and consequently, only the LED 101 (or 101') emits light. On the other hand, when the input light beams A and B are simultaneously applied, only those of the HPT's 100 and 100' which are not connected to the LED's 101 and 101' are turned ON, with a result that a current will not flow through either of the LED's 101 and 101' and hence they will not emit light. In the absence of the input light beams A and B, a current does not flow through none of the HPT'S, and hence they do not emit light. It will be seen from the above that the optical inputs A and B to the HPT's and the optical outputs C and D from the LED's bear a relationship just like an exclusive-NOR (XOR).
The conventional XOR optical logic element has a shortcoming that its operating speed is not so high, because it converts an optical signal to an electrical signal by phototransistors and then drives light emitting diodes of slow operating speed to obtain therefrom an optical output as mentioned above. Moreover, the prior art XOR optical logic element has a complicated structure, and hence calls for a complex manufacturing process.